1. Field of the Invention
The invention relates to aligning a paraboloidal off-axis mirror segment to a point source, and more particularly to improvements in the alignment procedure for an autocollimating test that uses a diverging beam (spherical wavefront) interferometer.
2. Description of the Prior Art
Perhaps the most frequently encountered aspheric optical surfaces are paraboloids or paraboloid segments. Their properties make them useful in a variety of telescopes and optical instruments.
The task of aligning any mirror surface interferometrically, whether for interferometric testing or for optical alignment within a larger system, requires some practice. For flat or spherical mirrors, the requisite level of skill is easily acquired by most competent technical professionals in a matter of minutes. For anyone dealing with aspheres, however, interferometric alignment can be quite challenging. All the alignment errors must be eliminated before the figure quality of the optic can be determined.
When off-axis paraboloidal OAP mirror segments and many other aspheric optics are interferometrically aligned, the configuration is usually in some form of null test. For paraboloids, the null test is often referred to as the autocollimating test. For paraboloids, two autocollimating tests exist. One uses an interferometer with a collimated (planar wavefront) beam in combination with a spherical reference surface; the other uses an interferometer with a diverging (spherical wavefront) beam in combination with a planar reference surface.
The invention improves the alignment of a paraboloid to a point in space, as required in the latter diverging beam type of autocollimating test. This is actually the more difficult of the two tests, but is often preferred for many applications for economic reasons and also because it can be used to test OAP segments of virtually any size.
The conventional diverging beam autocollimating test uses an interferometer, the off-axis parabolic segment and a reference flat. The diverging beam from the interferometer or lens is a spherical wavefront, and is precisely aligned such that the center of emerging wavefront is exactly coincident in space with the focal point of the paraboloid. The wavefront reflected from the parabolic segment is therefore collimated, a planar wavefront, and the reference flat is precisely aligned such that it is parallel to this wavefront (normal to the collimated beam).
The difficulty in achieving the precise alignment of these components occurs because the interferometer, OAP segment, and flat must be aligned as a system in prior technique. The most critical and demanding alignment is that of the paraboloid to the interferometer. Movement of the paraboloid relative to the interferometer, however, affects the entire system alignment. Thus, until the system is realigned by adjusting the reference flat, the effect of moving the paraboloid cannot be interpreted. The concurrent alignment of the two optical surfaces (paraboloid and reference surface) requires a high level of skill. A competent technical professional can expect to spend many hours to many days developing this level of skill. The level of success achieved is heavily dependent upon the individual's capabilities in the areas of geometry, spatial relations, and logic. Once skilled in the alignment process, the first time alignment of any "new" paraboloid or parabolic segment may still require several hours.